Snap buttons which attaches and detaches female snap buttons and male snap buttons are widely used for clothes, bags, and the like. The male snap button generally has a protrusion having a tip with a slightly larger diameter. By engaging and disengaging the protrusion with and from a protrusion receiving portion of the female snap button, the female snap button and the male snap button are connected and disconnected. There is known one type of the female snap button, in which an annular spring as a separate member is incorporated in a metallic female snap body, as shown in FIG. 6 of Japanese Utility Model Publication No. S63-90 A1. Such a spring is generally formed by cutting and bending a metal or resin wire member having a circular cross section. The female snap body is provided with a spring holding portion for holding the spring. When connecting the male snap button to the female snap button, the protrusion of the male snap button is inserted into the spring of the female snap button. At this time, the tip with a slightly larger diameter (a larger diameter portion) of the protrusion engages with the spring and elastically expands the spring radially outward, and as soon as the larger diameter portion passes across the spring, the spring is restored radially inward while holding the protrusion. This will lead to connection of the female snap button and the male snap button. Also in the case where both are disengaged, the larger diameter portion of the protrusion is engaged with the spring and the spring temporarily expands outward in the radial direction.
At present time, there is a need for a snap button as thin as possible in clothes and the like. However, the spring having the circular cross section of the female snap button as described above cannot reduce the size, i.e., the diameter so much, in order to maintain elastic deformation performance and holding force for engaging with the protrusion of the male snap button and holding it. Further, the spring holding portion of the snap body requires at least the thickness of the diameter of the spring (the length along the axial direction of the female snap button) to house such a spring. Therefore, when attempting to reduce the thickness of the entire female snap button, the spring itself and the spring holding portion requiring the thickness of the spring diameter have been bottlenecks.